Add BAL SmartFactor example

[dvorak0]

Summary:

• Add examples/SFMExample_SmartFactor_bal.cpp.
• It is a BAL-input version of SFMExample_SmartFactor.cpp:
  • reads data with SfmData::FromBalFile
  • builds SmartProjectionFactor graph
  • adds two camera priors to fix gauge/scale
  • runs LM and prints final error + iterations
• Goal: enable direct SmartFactor testing on BAL datasets (comparable to SFMExample_bal).

[dvorak0]

@dellaert hi prof. frank, could you have a look at this?

[dellaert]

LGTM !